As is known and recognized, nonlinear interactions such as cross-phase modulation (XPM) and self-phase modulation (SPM) directly modify the temporal phase of an optical field. As a result, long-haul optical transmission—where optical pulses are transmitted over long lengths of optical fibers—is impaired. The induced temporal phase shifts can also be used in applications such as pulse compression, regeneration, performance monitoring and photonic logic.
Accordingly, the ability to precisely determine the phase of an optical wave is a necessary requirement of many optical situations. Unfortunately, while the intensity of an optical wave may be obtained directly—using common square-law detectors—the phase of the optical wave can only be measured indirectly, as direct detectors are insensitive to optical phase.